Project Summary We have recently published that light entrainment of the local circadian clock in the retina is dependent on Opsin 5 (OPN5) and that Opn5 is expressed in a subset of retinal ganglion cells (RGCs). In preliminary analysis we have also shown, (1) that mice mutated for Opn5 or in clock genes (Per2 and Bmal1) have vascular development defects in both the retina and hyaloid vessels, and (2) using ChIP-seq analysis, that the vascular modulators Flt1 and EphrinB2 are directly regulated by the clock transcription factor BMAL1. Based on these data, we propose a model to explain the mechanism by which OPN5 regulates vascular development. In this model we suggest that light stimulation of OPN5 in RGCs first entrains a circadian clock in the neurons of the inner retina. We suggest that in turn, the clock transcription factor BMAL1 directly regulates rhythmic expression of Flt1 and EphrinB2. Since these are crucial regulators of the VEGFA (vascular endothelial growth factor A) response, this provides a direct link to the regulation of retinal angiogenesis and hyaloid vessel regression. Our central hypothesis is that an OPN5-dependent, local retinal clock regulates expression of Flt1 and EphrinB2 and, in turn, light-dependent vascular development of the eye. To investigate this hypothesis, we propose three aims. We will determine when during development the OPN5-dependent retinal clock is first active (Aim 1), whether retinal clock function is required for normal vascular development (Aim 2) and whether Flt1, EphrinB2 and other vascular mediators are regulated by an OPN5-dependent retinal clock. The existence of an OPN5-dependent pathway that regulates vascular responses in the eye is an unusual finding that may have important implications for the human vascular retinopathies.